Vol. 4 No. 2 Spring 2008 Journal of Double Observations Page 59

Visual Double Star Measurements with an Alt-Azimuth Telescope

Thomas G. Frey

California Polytechnic State University San Luis Obispo, CA 93407

Abstract: An alt-az mounted Newtonian telescope was used to determine the separation and position angle of seven known and five neglected double . The problem of field rotation was solved by modifying the usual observing technique. Separation and position angle determinations are described, and the standard deviations and mean errors for these measurements are presented. The direction of future studies is outlined.

techniques were accurate and precise, additional Introduction measurements on neglected double stars listed in the Professional astronomers have carried out visual Washington Double were made. double star measurements for over 200 . These scientists measured the separation between double Double Star Observation: Equatorial vs. stars in arc seconds, and the position angle in degrees Alt-Az Mounts that defined the orientation of pairs with respect to Most observers involved in double star measure- celestial north. Over time, the orbital motion of each ments, including Argyle (p.x) and Teague (p.112), star can create a change in the observed separation recommend the use of equatorial mounted telescopes. and position angle if the pair proves to be binary in Such telescopes have drive motors that are oriented so nature. A revolves around a common the axis rotates around the north center of mass. celestial pole, canceling out the Earth’s rotation and Today’s amateur astronomers continue to evaluate the image in the eyepiece remains stationary. these changes with fairly simple equipment. The The equatorial telescope can be equipped with an usual recommended setup includes an equatorial illuminated reticle eyepiece such as the 12.5 mm mounted telescope with tracking motors, a laser- Celestron Micro Guide or the 12 mm Meade astromet- etched astrometric eyepiece, and a stopwatch. ric eyepiece. Both eyepieces have similar configura- This study, however, utilizes an altitude-azimuth tions: a linear scale in the middle and a 360° protrac- (alt-az) mounted telescope instead of an equatorial tor scale around the circumference of the field of the mounted telescope. Alt-az mounts are seldom used in eyepiece. The linear scales are divided into 60 and 50 double star measurements due to the rotation ob- equal divisions on the Celestron and Meade eyepieces, served in the field of view. This motion can affect both respectively. Sometimes an external protractor scale the accuracy and precision of the measurement of the is mounted to the base of the eyepiece to more accu- position angle. Yet, with minor adjustments made at rately measure the position angles, as described by regular intervals during the observing session, both Tanguay (p.116) and Johnson and Genet (p.147). separation and position angle measurements made on Separation between double stars is determined by known double stars correlate closely to literature using the slow motion control of the equatorial tele- values. Once it was determined that the measuring scope to align the pair on the linear scale. Then esti- Vol. 4 No. 2 Spring 2008 Journal of Double Star Observations Page 60

Visual Double Star Measurements with an Alt-Azimuth Telescope mate the number of divisions on the scale between the larCAT. This system is controlled with a Wildcat Argo centers of the stars to the nearest tenth. The number Navis computer. The Meade 12.5 mm astrometric of arc seconds represented by each scale division is eyepiece was used for all double star measurements. A previously determined by using either the drift RadioShack LCD Stopwatch with 0.01 second resolu- method with a star of known celestial coordinates, or tion was used to calibrate the linear scale of the calibration double stars alone, that have had no Meade eyepiece. change in separation in 50 years. The observation sessions were held in San Luis Position angles are measured again by using a Obispo, CA and Atascadero, CA. San Luis Obispo is slow motion control, this time to move the primary located at latitude 35°16’ N, altitude 360 feet, in an star to the center of the linear scale. The eyepiece is area where light pollution is extensive and an evening rotated until the secondary star is also on the linear marine layer limits the time of effective seeing. Atas- scale. The drive motors are turned off and the pair cadero is located approximately 15 miles north of San drifts across the field of view until the primary Luis Obispo at 35°30’N, altitude 1050 feet, in an area reaches the protractor scale at the outer edge of the of limited light pollution and no marine layer. field. The drive motors are re-engaged and the posi- tion angle is indicated by the position of the primary Calibration of the Meade Astrometric star on the protractor scale. Eyepiece The axis of a telescope attached to an alt-az mount The linear scale of the reticle eyepiece must be rotates about the zenith, not the celestial pole like an calibrated to the telescope being used. To determine equatorial mount. Even if the alt-az telescope is the number of arc seconds per division, Argyle (p. 152) equipped with drive motors, stars in the field of view recommends using a star of medium brightness rotate around objects in the center of the field. The (magnitude 5-6) with a of 60-75° and closer a celestial object is to the zenith, the faster the allowing it to pass along the length of the linear scale. rotation in the field. This rotation makes any type of This is carefully timed to the nearest 0.01 seconds. To imaging (astrophotography or CCD imaging) problem- reduce random errors in the process, 8-10 different atic. It also can affect double star measurements, drift times were recorded and the average determined. especially position angle determination. Field rotation The average drift time is used in the following can be compensated for by using a de-rotator attached equation: to the eyepiece focuser, but this requires an additional motor and adds additional complexity and expense to 15.0411T cos(δ ) Z = avg RS the instrumentation. D Argyle (p. 286) and Napier-Munn (p. 22) both point out that the rate of field rotation observed in alt- where Z is the scale constant in arc seconds per az mounted telescopes is greatest at the zenith and division, Tavg is the average drift time of the reference zero when a star crosses the prime vertical, that is, star across the scale in seconds, 15.0411 is the side- when the star is due east or due west. As a result, real motion in arc seconds per second of Earth’s rota- initial double star measurements were conducted in tion, cos(δRS) is the cosine of the declination of the an easterly direction and between altitudes of about reference star, and D is 50, the number of divisions for 20-80°. the Meade eyepiece It will be shown in this study that if simple ad- Both Alpha Cephei (Alderamin) and Gamma justments are made to the reticle eyepiece of an alt-az Cassiopeiae (Navi) were used as calibration stars, telescope during double star data collecting, excellent depending on the time of the the observations results can be achieved in separation and position were conducted. Typical examples of the calibrations angle measurements that closely agree with data are indicted in Table 1. The units for the scale con- obtained with traditional equatorial telescopes. stant are arc seconds per division (a.s./div). Equipment Used Procedure for Measuring Separation The telescope used in this research was an Obses- After determining the scale constant for the sion f/4.5 18-inch Newtonian telescope with a Dob- Meade astrometric eyepiece, the telescope was two- sonian (or alt-az) mount. It was equipped with a star aligned and the tracking motors engaged. The ServoCAT tracking and GOTO system made by Stel- primary star was centered in the eyepiece and the Vol. 4 No. 2 Spring 2008 Journal of Double Star Observations Page 61

Visual Double Star Measurements with an Alt-Azimuth Telescope

Bess. Dec. Av Drift Scale Constant Star # Obs. Std. Dev. Mean Error (°) Time(sec) (a.s/div)

α Cephei B2007.618 62.58 16 88.17 0.283 0.071 12.21

γ Cass B2007.769 63.67 12 83.49 0.280 0.081 12.22

Table 1: Determination of the scale constant eyepiece rotated so that both stars were co-aligned on Final Procedure Adopted the linear scale. The number of divisions between the Instead of doing repetitive two-star alignments stars was noted and estimated to the nearest 0.1 with the Argo Navis computer after every position division. The slow motion control on the ServoCAT angle measurement, the tracking motors were disen- was then used to move the double star along the scale gaged after the separation measurements were con- to a new location and another reading recorded. This ducted. This allowed manual movement of the tele- was done to reduce the random error in assigning the scope. The eyepiece was rotated until the double stars number of divisions. Usually, no fewer than 10 read- were aligned on the linear scale. Then the telescope ings were recorded and the average number of divi- was moved so that the primary star accurately drifted sions calculated. The separation in arc seconds was through the central division mark. In practice, the calculated by multiplying the scale constant, Z, by the primary was situated about 5-8 division marks away average number of divisions between the double stars. from the central mark and allowed to drift. If the star Procedure for Measuring Position Angle drifted through the central mark, the drift sequence was allowed to continue until the primary star passed Initial Attempts across the outer protractor scale, and the angle re- Initial position angle measurements mimicked corded. If it missed the central mark, the scope was procedures used with equatorial mounted telescopes. moved and another pass was attempted. This was The slow motion controls were used to center the much more successful and efficient than the former primary, or brightest, star of the double star on the method. Also, after the position angle was recorded, center mark of the linear scale, i.e. on the 25th divi- the telescope was moved immediately until the star sion. Due to the presence of backlash, this was tedi- was repositioned in the center of the field of view. ous, frustrating, and not always successful. Then the The reticle eyepiece was rotated 180° after every eyepiece was rotated, ensuring the stars were aligned other drift measurement to allow constant realign- on the linear scale. This sometimes resulted in having ment of the double stars with linear scale. Of course to re-center the primary star. If successful alignment 180° had to be subtracted from half of the measure- was achieved, the servo-motors were disengaged and ments so correct position angles were evaluated. This the double star allowed to drift to the edge of the eyepiece rotation cancelled random errors that could eyepiece. As the primary crossed the outer protractor have occurred if the eyepiece had been left in a single scale, the position angle was noted and recorded. orientation. The eyepiece was rotated 180° to prevent random The majority of the known and neglected double errors of alignment. Then the procedure of two-star stars selected for this study had separations greater alignment was repeated using the Argo Navis com- than 34 arc seconds. Only one double star, Eta puter, the primary star centered in the eyepiece, the Casseopeiae, with an observed separation of 12.8 arc secondary star aligned on the linear scale, and the seconds, was included, since this was one of the initial motors disengaged. This procedure was time consum- trials performed. It was determined that any separa- ing, especially the centering of the primary star. It tion spanning less than 3 divisions on the linear scale was tempting to use an off-centered primary that was made it very difficult to align the stars for position “close enough”, which would have resulted in an angle drift. If double stars with separations less than erroneous position angle. It was necessary to find about 30 arc-seconds were to be studied, a Barlow lens another technique of accurately centering the primary should be employed. Indeed, Napier-Munn (p.25) star. found the use of a Barlow problematical except with Vol. 4 No. 2 Spring 2008 Journal of Double Star Observations Page 62

Visual Double Star Measurements with an Alt-Azimuth Telescope

Separation(arc seconds) Position Angle(degrees)

Double Bess. # SD/ME Obs. Lit. ΔSep. # SD/ME Obs. Lit. ΔPA Lit. Star Epoch Obs. Sep. Sep. Obs. PA PA Epoch Pi And B2007.670 5 0.04/0.02 36.9 36.4 +0.5 9 1.31/0.44 171.4 175 -3.6 WDS2006

Eta Cas B2007.673 4 0.06/0.03 12.8 12.9 -0.1 4 1.93/0.97 315.6 319 -3.4 WDS2005

Table 2: Separation and Position Angle for Pi Andromedae and Eta Cassiopeiae

separations of 3 divisions or less on the reticle. All (SD) and mean errors (ME) for the number of observa- observations in this study used just the Meade astro- tions recorded. metric eyepiece alone. The values for separation and position angle obtained in the current study for Mu Herculis and Results and Analysis of Observations STF 698AB closely compare to other recent observa- Known Double Stars tions. See Tables 4 and 5. Table 2. lists the results obtained for the two Neglected Double Stars initial double stars studied. These observations were performed in San Luis Obispo, CA. Due to a lingering Five neglected double stars were measured at Hill marine layer and light pollution in the city, only House Observatory. Table 6 lists the results of the bright double stars were investigated. Pi Andromedae, separation and position angle measurements, respec- tively, along with their statistical evaluations. m1:m2, 4.3:7.1, and Eta Cassiopeiae, m1:m2, 3.5:7.4, were appropriate initial studies, except for the fairly Conclusions small separation for Eta Cassiopeia. The observed The purpose of this study was to attempt separa- separation and position angle are listed with their tion and position angle measurements of known and corresponding standard deviations (SD) and mean Δ Δ neglected double stars with an alt-az mounted tele- errors (ME). The columns headed Sep and PA scope. Prior investigations strongly recommended indicates the differences between the observed and equatorial mounted telescopes be used for such stud- literature values for separation and position angle, ies due to the field rotation observed in alt-az respectively. mounted telescopes. It was suggested that separation The telescope was moved farther from the effects measurements could be easily and accurately per- of coastal fog and light pollution of San Luis Obispo 15 formed using the ServoCAT/Argo Navis control sys- miles north to higher elevation and drier, darker skies tem with an alt-az telescope. in Atascadero, CA at Hill House Observatory. Longer Initial position angle measurements were at- observation sessions with steadier skies were possible tempted by using the handpad controls on the Servo- due to this move. Five additional known double stars CAT/Argo Navis system to center the primary star, were measured at this location. Table 3 lists the but the jerky backlash that occurred made it difficult results of the separation and position angle (PA) and frustrating to try to center the primary star on measurements, along with the standard deviations

Separation(arc seconds) Position Angle(degrees)

Double Identifer Bess. Epoch # SD/ME Obs. Lit. # SD/ME Obs. Lit. Lit. Star Obs. Sep. Sep. Obs. PA PA Epoch Tau Tau 04422+2257 B2007.769 8 0.05/0.02 62.7 63.0 10 1.11/0.35 213.2 214 WDS1999

Mu Her 17465+2743 B2007.771 10 0.19/0.06 34.9 34.9 13 2.79/0.77 248.2 249 WDS2000

Beta Cam 05034+6027 B2007.793 18 0.10/0.02 85.3 83.0 13 0.92/0.26 208.3 210 WDS2003

STF 698AB 05252+3451 B2007.769 8 0.18/0.06 31.2 31.2 14 1.50/0.40 349.8 347 WDS2004

FRK 11 22301+4921 B2007.793 9 0.11/0.04 67.2 66.4 10 1.23/0.39 91.8 91 WDS1998

Table 3: Separation and Position Angle for Five Known Double Stars Vol. 4 No. 2 Spring 2008 Journal of Double Star Observations Page 63

Visual Double Star Measurements with an Alt-Azimuth Telescope

Year Separation Position Angle Source Year Separation Position Angle Source

1973 35.00” 248.0° Comellas 1970 31.00” Comellas 346.0°

1980 35.00” 249.0° Comellas 1980 31.00” 346.0” Comellas

1994 29.30” Rojo 1987 32.00” 247.0° Schnabel 348.0° 2007 31.2” Frey 2007 34.9” 248.2° Frey 349.8°

Table 4: Separation and Position Angle for Mu Herculis Table 5: Separation and position angle for STF 698AB since since 1973 1970 the middle division of the linear scale of the reticle may be explained by sky conditions. The double star eyepiece. When the servo-motors were disengaged and was observed in the evening at the low altitude of 16° the primary star manually moved into a position and it was windy. where the star could drift through the center division, Five neglected double stars were then observed. accurate and precise position angles could be repeat- The differences and percent difference between ob- edly recorded. served and literature values for these double stars are To verify that alt-az mounted telescopes could be given in Table 8. Negative values indicate the ob- used to make accurate measurements on double stars, served value was less than the literature value; posi- seven double stars with known fixed separations and tive values indicate a greater value than literature. position angles were observed and compared to litera- Again, the low percent differences for most of the ture values. The differences in separation and position neglected double stars in Table 8 demonstrate the angles (DSep and DPA, respectively) and the percent accuracy of the alt-az system. It should be noted that differences between the observed and literature val- percent differences for BUP 91AC and STT 23AC ues for five of the seven known double stars studied separations are prominently larger than the other are given in Table 7. Negative values indicate the neglected double stars studied. The reason for this observed value was less than the literature value; difference was pointed out by Dave Arnold (personal positive values indicate a greater value than litera- communication). Arnold stated that BUP 91AC and ture. STT 23AC are both composed of multiple star sys- The low values shown in Table 7 indicate that the tems. For STT 23 the A and B components are in observed values for separation and position angles are common but the C component is a very close to the literature values demonstrating that background star having no physical connection to A alt-az mounted telescopes can be used for accurate and B. So the difference in separation between A and and precise measurements of double stars. The larger C in the current study (50.1”) to that observed in 1909 values for difference and % difference for Beta Cam (56.97”) is due to proper motion from component A,

Separation(arc seconds) Position Angle(degrees)

Double Identifer Bess. # SD/ME Obs. Lit. # SD/ME Obs. PA Lit. Lit. Star Epoch Obs Sep. Sep. Obs. PA Epoch

BU 492AC 00453+5513 B2007.834 10 0.16/0.05 89.1 88.6 13 0.81/0.22 24.7 26° WDS2000

BUP 91AC 06377+6129 B2007.840 10 0.16/0.05 393.1 379.9 12 0.58/0.17 91.7 94° WDS1929

STF 10AC 00148+6250 B2007.840 11 0.13/0.04 55.9 55.7 10 0.26/0.08 102.3 101° WDS2000

STT 23AC 01101+5145 B2007.840 10 0.24/0.07 57.0 50.1 12 1.08/0.31 92.2 94° WDS1909

STF 70AC 00538+5242 B2007.848 11 0.07/0.02 73.5 75.3 12 0.83/0.24 154.0 151° WDS1985

Table 6: Separation and Position Angle Measurements for Five Neglected Double Stars Vol. 4 No. 2 Spring 2008 Journal of Double Star Observations Page 64

Visual Double Star Measurements with an Alt-Azimuth Telescope

Double D Sep Sep % D PA PA % exactly where to position my eye to get precise Star (arc secs) Difference (degrees) Difference results. Tau Tau -0.3 -0.48 -0.8 -0.37 New Directions with an Alt-Az Mu Her 0 0 -0.8 -0.32 Telescope Beta Cam 2.3 2.73 -1.7 -0.81 At Hill House Observatory, the lower limit of STF 698AB 0 0 2.8 0.80 visual perception was estimated to be magnitude 12. Such stars could only be seen using averted FRK 11 0.8 1.20 0.8 0.88 vision. This was most challenging when position Average 0.56 0.69 0.06 0.03 angle measurements were attempted. The star would “disappear” behind the main line of the Table 7: Differences and % Differences between Observed and Literature linear scale. To visualize the star, one had to look Values for Separation and Position Angles of Five Known Double Stars about 5-8 division marks above or below the linear scale and estimate the corresponding measurement. Two ways of solving this problem come to mind. not by orbital motion. First, although Hill House was excellent for He also states that BUP 91AC form an optical double star observation for magnitude 10 and less, a double star, because the proper motion vectors of darker site would improve the ability of seeing these these stars diverge. The values of the A component very faint stars. There was still some light pollution are -208 mas/yr (milliarc seconds/ year) in right ascen- from the city lights of Atascadero. sion and -275 mas/yr in declination. The values for the Second, use of the Celestron Micro Guide eyepiece C component are +8 and -25 mas/yr, respectively. instead of the Meade astrometric eyepiece is sug- Again, this was due to the proper motion of component gested. The Celestron model has two parallel lines A compared to the background star, C. running the length of the linear scale instead of just Standard deviation and mean error analysis for the single line on the Meade model. Faint stars would drift times across the linear scale, estimated divisions be more visible between the two lines than behind or between the double stars, and drift positions meas- on a single line. ured on the protractor were calculated. These statis- The light generated by the illuminated reticle can tics, summarized in Table 9, were used in determining also prevent the detection of faint stars. The eyepiece the scale constants, separations, and position angles has an adjustable switch that can vary the amount of (PAs), respectively. Except for an elevated standard illumination desired. But it can only be dimmed to a deviation value for the position angle of the known certain level. This minimum intensity may still inter- stars, all other statistics have very low values, indicat- fere with seeing faint stars. This problem can be ing a fairly precise series of observations. solved by purposely using weak batteries or by placing The larger standard deviations for the position a piece of aluminum foil between the battery and the angles of the known double stars are probably the results of the author’s Double D Sep Sep D PA PA initial inexperience in making these Star (arc secs) % Difference (degrees) % Difference measurements. The drift method used BU 492AC 0.5 0.56 -1.3 -5.13 for position angle measurement took quite a bit of practice to position the BUP 91AC 13.2 3.42 -2.3 -2.48 telescope so the primary star drifted through the exact center of the eyepiece. STF 10AC 0.2 0.36 1.3 1.28 Initial attempts were slightly off the STT 23AC 6.9 12.89 -1.8 -1.93 mark. Also, as the primary star drifted across the field of view and crossed the STF 70AC -1.8 -2.42 3.0 1.97 outer protractor scale, a parallax was observed. Different values of the posi- Average 3.8 2.96 -0.22 -1.26 tion angle were detected depending on how you looked through the eyepiece. It Table 8: Differences and % Differences between Observed and Literature Values took several sessions before I knew for Separation and Position Angles of Five Neglected Double Stars Vol. 4 No. 2 Spring 2008 Journal of Double Star Observations Page 65

Visual Double Star Measurements with an Alt-Azimuth Telescope

Double Star SD for all SD for all ME for all ME for all Type separations PAs separations PAs Known <0.20 <2.80 <0.07 <1.00

Neglected <0.25 <1.10 <0.08 <0.32

Table 9: Standard Deviation (SD) and Mean Error (ME) Trends for Separation and Po- sition Angle Measurements Determined for Known and Neglected Double Stars. electrical contact on the battery case. Both methods telescope in the much drier and darker site at Hill will reduce the illumination of the reticle eyepiece. House Observatory and for help generating many star Double stars with separations greater than 30” charts. I want to thank Tom Smith of Dark Ridge were chosen for the current paper. It is difficult to get Observatory in New Mexico for sending me informa- repeatedly accurate alignment of the double stars on tion on double stars and helping me decipher the the linear scale for position angle measurements of double star catalogs. Finally, I really appreciate the less than three divisions. To measure double stars help provided by Russ Genet, Jo Johnson, Vera with smaller separations than 30”, a Barlow lens Wallen, Tom Smith, Dave Arnold, R. Kent Clark, Bob should be employed. A 2x or 5x Barlow will magnify Buchheim, and Jim Carlisle for reviewing this paper the separation and make division estimates easier. and for all their suggestions. A special thanks goes to However, if the seeing is marginal, as it often is in Dave Arnold for his insight on the neglected stars coastal regions, the Barlow will magnify the seeing BUP 91AC and STT 23AC. problems as well. Such studies should be done at dark sky sites when the sky is very steady. References Acknowledgements Observing and Measuring Visual Double Stars, Robert Argyle, Springer, London 2004. There are many people I want to thank for assis- tance in this study. Russ Genet, Professor of Astron- Johnson, Jolyon M. and Genet, Russell M., Fall 2007, omy at Cuesta Community College and Research Journal of Double Star Observations, 3, 147-150. Scholar in Residence at California Polytechnic State Napier-Munn, Tim, 2007, The Webb Society Double University, recently showed me that an 18” alt-az Star Section Circulars, 15, 15-28. Newtonian telescope can be used in scientific research of double stars and is not just relegated to observation Tanguay, Ronald Charles, 1999, Sky and Telescope, of deep sky objects at monthly club star parties. Whole February, 116-120. new frontiers are now open to those with alt-az tele- Teague, Thomas, 2000, Sky and Telescope, July, 112- scopes, thanks to his insight. A big thanks goes to Jim 117. Carlisle and wife Pat for allowing me to set up my

Thomas G. Frey is Professor Emeritus of Chemistry at California Polytechnic State University in San Luis Obispo, CA. He has been an active member of the Central Coast Astronomical Society for over 25 years.